1. Field of the Invention
This invention relates to improvements in uniformity in the magnetic gap field distribution in a magnet device which is used in an analyzing apparatus for analyzing such as nuclear magnetic resonance, and more particularly to improvements in a pole piece for use in the aforesaid magnet device and a method for manufacturing same.
2. Description of the Prior Art
An atomic nucleous or electron has a magnetic moment due to its rotation on its axis. Thus, when the atomic nucleus or electron is placed in a high magnetic field which has been generated by a magnet and oriented in a given direction consistently, then there takes place polarization in the direction of the magnetic field thereof. If a high frequency magnetic field is applied to the atomic nucleus or electron in the polarized condition in the direction at a right angle to the polarizing direction by means of coils, then the nucleus or electron precession occurs in the magnetic field, thereby causing rotation of a magnetic moment having the same angular velocity as that of the high frequency magnetic field applied. As a result, there takes place variation in voltage between the coils, on which the high frequency magnetic field has been impressed, so that the aforesaid variation in voltage may be detected as signals.
An analyzing apparatus for analyzing nuclear magnetic resonance utilizes the aforesaid principle to detect the condition of the atomic nucleus or electron in a material. This apparatus is known as being advantageous for clarifying the bonding condition of atomic nucleus or electron or the molecular construction of a compound, because of its extremely high resolving power.
A circular frequency .omega. for magnetic resonance is given by .omega. = .gamma.H, wherein .gamma. represents a gyromagnetic constant and H represents the intensity of a polarized magnetic field. As can be seen from this formula, if the magnetic field in a sample space becomes uneven, then there results variation in magnetic resonance frequency in the respective portions of the sample, so that the range of resonance signals will be broader, with the resulting reduction in resolving power. On the other hand, the fact that the polarized magnetic field is intense signifies that the resonance frequency is high, thus presenting advantages from viewpoints of the separation of frequency and the signal-to-noise ratio. Accordingly, a magnet device which provides extremely uniform and strong magnetic field is required for the unclear magnetic resonance analyzing apparatus.
In the practical application, there has arisen a demand for magnet device which presents a magnetic-gap-field instensity ranging from several thousands Oe to several hundred thousands Oe, as well as a uniform magnetic field having a variation of less than 1 .times. 10.sup..sup.-5.
A pole piece is used for such a magnet device for the purpose of collecting magnetic fluxes in the gap portion of a magnet to thereby increase the intensity of a magnetic gap field. It is a common practice to use as a material for a pole piece a magnetic material having a high magnetic flux density and magnetically uniform composition. According to the prior art method for manufacturing such a pole piece, the starting material is melted to provide an ingot, and then the ingot thus prepared is subjected to hot forging and hot rolling to thereby provide a billet. Then, the billet is machined by means of a lathe to a shape of the pole piece desired, followed by heat treatment. The magnetic properties of the pole piece thus obtained is uniform throughout the pole piece, and such properties have been required.
The uniformity in a magnetic gap field depends on the surface-magnetic-charge distribution, while it also depends on the magnetic properties of a material use, the shape of a pole piece, i.e., the gap/diameter ratio, and the tapered angle of the tip of the pole piece.
With the conventional pole piece, the diameter of the pole piece is increased for enhancing the uniformity in the magnetic gap field, because the uniformity in the magnetic field of a space confined by the parallel surfaces having infinite areas is ideal. For those reasons, a pole piece of a size excessively large for a sample space, for instance, the pole piece having a diameter of 200 mm, is used for the sample space of 5 .times. 5 .times. 5 mm. The increase in size of the pole piece results in an increase in size of a magnet, i.e., a magnetic-motive-force-generating portion, so that the weight of the entire magnetic device is increased to as high as 4 tons.
As an alternative, there has been proposed a method for improving uniformity in a magnetic field, wherein a magnetic ring is fitted on the outer periphery of the pole piece. However, such a method suffers from disadvantages that there is considerable discontinuity in the surface-magnetic-charge distribution on the boundary of the aforesaid fitted portion, with the result that there takes place microscopic non-uniformity in the magnetic gap field, thus failing to present desired uniformity for magnetic field.
Still alternatively, there has been proposed a method, by which to provide a spherical surface for a pole piece. However, the polishing level required for the spherical surface is far from practicality.